This invention relates to an apparatus for detecting the outflow of molten slag placed on the surface of a molten metal, usually molten steel, during continuous casting. In particular, this invention relates to an apparatus for detecting the outflow of molten slag into a tundish or a casting mold through a refractory sealing member when the molten metal is being poured from a ladle into a tundish or from a tundish into a casting mold under sealed conditions in a continuous casting process.
After finishing steel refining in a converter, the resulting molten steel is stored in a ladle and then carried to a continuous casting apparatus, where the molten steel is poured by way of a tundish into a mold and solidified into semi-finished products such as slabs, blooms or billets. While molten steel is in the ladle, the surface thereof is covered by a floating molten slag which serves to suppress the oxidation of the molten steel and also to maintain the temperature thereof If the molten slag flows out into a tundish together with the molten steel, the slag will be entrained and poured into a casting mold together with the molten steel. The thus entrained slag forms non-metallic inclusions in the cast slab, degrading the quality thereof. Therefore, in view of the necessity of quality control of cast slab and casting operation control, it is desirable to detect the outflow of slag through a nozzle from a ladle into a tundish or from a tundish into a casting mold in order to reduce as much as possible the amount of slag entrained into cast slabs. The pouring from a ladle into a tundish is stopped at once when the slag outflow is detected.
The conventional methods used to detect the outflow of slag include, other than a visual observation method, a direct-sight method employing a radiant heat thermometer (Japanese Patent Application Laid-Open No. 72752/1982), a method of measuring a change in the impedance of coils surrounding a nozzle down which molten steel flows, the change in impedance being caused by the difference in electrical conductivity of molten steel and molten slag which flow down through the nozzle (Japanese Patent Application Laid-Open Nos. 64961/1980, 122656/1981 and 167066/1983), and a method of measuring the vibration of a tundish and nozzle (Japanese Patent Application Laid-Open Nos. 97846/1980 and 97847/1980).
Recently, a sealed casting process has been proposed for the purpose of reducing the amount of inclusions in a cast slab, and in this process a poured stream of molten steel from a ladle into a tundish is covered or surrounded by a refractory sealing member and the inside thereof is filled with an inert gas such as Ar gas to suppress oxidation of the poured flow of molten steel. Alternatively, a long nozzle of the immersing type is provided at the bottom of a sliding nozzle and the opposite end of this immersing-type nozzle is plunged into molten steel in a tundish so as to avoid contact between the poured stream of molten steel and atmospheric oxygen. The sealed casting process is therefore effective to suppress oxidation of molten steel so as to reduce the amount of inclusions in a cast slab. In case the sealed casting process is employed, however, it is impossible to apply a visual observation method and a direct-sight method to detect slag outflow, because a poured stream of molten steel or slag is not exposed to the atmosphere and cannot be seen directly. In addition, the nozzle for use in the sealed casting process is made of alumina graphite which exhibits a high level of strength and heat resistance and also a remarkable resistance to heat erosion. However, alumina graphite has a high level of electrical conductivity, and the above-mentioned coil impedance measuring methods cannot be applied.
The inventors of this invention disclosed in Japanese patent application Laid-Open No. 121864/1982 a process to detect the outflow of slag by means of measuring the thermal radiation energy at the microwave frequencies by a radiometer (i.e. high sensitive receiver) to determine the outflow of molten slag on the basis of the difference in the emissivity of molten steel and of molten slag when they flow out from a ladle or a tundish through a refractory sealing member.
However, according to the above-mentioned method, the thermal radiation energy is received by an antenna in the shape of horn and it is necessary to place the antenna as close to the refractory sealing member as possible.
It is the current trend to employ a continuous casting process of the multi-strand type. When such a multi-strand process is employed, it is necessary to frequently and quickly replace or change the ladle and tundish, and sampling operations are also required frequently, and therefore, many operations must be carried out near the ladle and tundish. This means that the method proposed above of detecting outflow of slag is impractical, because it is necessary to install an antenna in the shape of a horn very near the nozzle (usually 30-50 mm away from the nozzle). The installation of this equipment interferes with the operations of changing ladles or tundishes and the operation of other apparatuses or appendixed devices. Therefore, it has been thought that it is difficult to install such apparatuses near a continuous casting apparatus, especially one employing the sealed casting process of the multi-strand type.
In addition, even in the afore-mentioned conventional methods, such as a direct-sight method, a coil-impedance measuring method and a vibration-measuring method, a radiant thermometer, coil, vibration sensor, etc. have to be installed near a measuring target point, i.e. a target point of measurement where a stream of molten steel flows down.
Therefore, there is a great need in the industry for an apparatus through which the outflow of slag can be detected at a distance of at least a few meters away from the measuring target point.